The present invention relates to a program shutter and, more particularly, to a program shutter in which sectors are opened and closed by means of a stepping motor.
A program shutter in which sectors are opened and closed by means of a stepping motor is arranged such that, in response to the operation of a release button, driving pulses having a predetermined period are input to the stepping motor so as to cause it to rotate forwardly, thereby gradually opening the sectors, and at the time when the motor has rotated through a number of steps corresponding to a particular amount of exposure, the phase of the driving pulses is switched to reverse the stepping motor so as to close the sectors, thus obtaining a correct exposure.
The shutter wherein the sectors are opened and closed by means of a stepping motor as described above suffers from the following problems. When the shutter is in a stationary state, a sector opening and closing member is forced to remain at a reference position by means of a spring in order to prevent occurrence of an erroneous operation. Accordingly, in the process of opening the sectors, the load of the spring is applied to the stepping motor. In addition, the change of the aperture diameter with respect to the elapsed time in the process of opening the sectors is non-uniform due to the inertia of components or the fit clearance therebetween. For these reasons, the prior art is incapable of effecting precise exposure thereof.